1. Field of the Invention
The present invention relates to an apparatus for steering the front and rear wheels of a motor vehicle such as an automobile, and more particularly to an improvement in a rear wheel control means in such a front and rear wheel steering apparatus.
2. Description of the Relevant Art
There is known an apparatus for steering the front and rear wheels of a motor vehicle while the ratio of a rear wheel steering angle to a front wheel steering angle is being variably controlled, as disclosed in British Pat. No. 2,044,699. In the known steering apparatus, when the motor vehicle runs at a low speed, the rear and front wheels are steered in opposite directions, thus allowing the motor vehicle to make a small radius turn. When the motor vehicle travels at a high speed, the rear and front wheels are steered in the same direction to make the motor vehicle highly maneuverable.
The conventional front and rear wheel steering apparatus of the type described has a speed sensor for detecting the speed of the motor vehicle, a steering angle sensor for detecting the steering angle of the front wheels, a control unit, and a drive motor for steering the rear wheels. The control unit is responsive to data of the vehicle speed and the front wheel steering angle, as detected by the sensors, for determining an optimum value for the rear wheel steering angle and controlling the drive motor so that the rear wheels will be steered through the determined optimum steering angle.
More specifically, the drive motor for steering the rear wheels is controlled by applying a control voltage proportional to the difference (hereinafter referred to as a "control error") between a target steering angle value and a present rear wheel steering angle (hereinafter referred to as a "present value"), to the motor, and reducing the motor control voltage as the control error decreases until the control error is eliminated, whereupon the motor control voltage applied to the motor becomes zero and the rear wheel steering angle reaches the target value. The above control system is known as a proportional control system since the ratio of the motor control voltage to the control error is constant as shown in FIG. 6 of the accompanying drawings. This proportional control system has been employed in conventional front and rear wheel steering apparatus.
As described above, the motor vehicle with steerable front and rear wheels has various detectors such as a steering angle sensor and a speed sensor for detecting the effects of driving operations of the driver on the behavior of the motor vehicle. Specifically, the steering angle sensor is employed to detect the amount and direction of steering as a result of turning the steering wheel, and the speed sensor is employed to detecting the speed of the motor vehicle as a result of operation of the accelerator pedal or brake pedal.
The conventional front and rear wheel steering apparatus has the following problems since the control system thereof is a simple combination of the detecting means and the control unit.
As shown in FIG. 7 of the accompanying drawings, the detecting means includes a sensor unit comprising a rotation detector and a signal output circuit which are connected to a power supply via power lines. Where the detecting means is a steering angle sensor unit, the rotation detector detects a steering angle from the steering wheel or a front wheel. Where the detecting means is a speed sensor unit, the rotation detector detects a motor vehicle speed from a wheel or a transmission. The signal output circuit receives a signal from the rotation detector and applies the detected signal over a signal line to the control unit in which the signal is processed.
The system arrangement shown in FIG. 7 suffers from the following possible failures:
(i) Failure of the rotation detector; PA1 (ii) Failure of the signal output circuit; PA1 (iii) Failure of the power supply and/or the power supply line; and PA1 (iv) Failure of the signal line.
In a case where the steering angle sensor unit is used as the detecting means, steering angle data produced at the time of a failure thereof is constant or remains unchanged as observed from the control unit or signal processor. The steering angle data is also constant or remains unchanged when the steering wheel is kept at a certain angle. Therefore, it would be impossible to ascertain whether the steering angle sensor unit is functioning or not, dependent on whether the steering angle is changed or not. Where the speed sensor unit is employed as the detecting means, a constant voltage is present on the signal line with no pulse signal produced in the event of a failure thereof, as observed from the signal processor. The voltage on the signal line also remains constant and no pulse signal is issued when the motor vehicle is at rest. As a result, it would not be possible to tell whether the speed sensor unit is functioning well or not on the basis of whether there is a pulse signal or not on the signal line.
In the event that the speed of the motor vehicle with the above front and rear wheel steering apparatus is abruptly reduced by the driver while the motor vehicle is being steered, the steering angle of the rear wheels is also abruptly varied as a function of the abrupt change in the motor vehicle speed, thus undesirably causing the running direction of the motor vehicle to vary significantly.
Moreover, because of the proportional control system employed, the conventional front and rear wheel steering apparatus has the following drawbacks:
(i) If a proportionality coefficient (i.e., the ratio of the motor control voltage to the control error) is increased in order to increase a control speed (i.e., to shorten a control delay time), then hunting occurs in a range in which the control error is small;
(ii) Conversely, if the coefficient is reduced in order to prevent such hunting, the control speed is lowered inasmuch as the motor control voltage does not become high enough in a range in which the control error is large.
More specifically, while any steering delay of the rear wheels at low speeds does not cause an appreciable problem, such steering delay adversely affects vehicle maneuverability during high-speed travel of the motor vehicle. If a control output for steering the rear wheels were increased so as to eliminate the steering delay thereof, then hunting would take place when the amount of controlling the steering angle is small.
The conventional front and rear wheel steering apparatus therefore fails to sufficiently control some motions since it merely detects effects of driver operations on the motor vehicle behavior and simply controls the rear wheels based on the detected effects, as described above. It has, therefore, been desired in the art to provide sufficient control over all such motions, which have not been controlled well in the past.